A Note on Lower Bounds for Induced Ramsey Numbers

Abstract

We say that a graph $F$ strongly arrows a pair of graphs $(G,H)$ if any 2-colouring of its edges with red and blue leads to either a red $G$ or a blue $H$ appearing as induced subgraphs of $F$. The induced Ramsey number, $IR(G,H)$ is defined as the minimum number of vertices of a graph $F$ which strongly arrows a pair $(G,H)$. We will consider two aspects of induced Ramsey numbers. Firstly there will be shown that the lower bound of the induced Ramsey number for a connected graph $G$ with independence number $\alpha$ and a graph $H$ with clique number $\omega$ roughly $\frac{\omega^2\alpha}{2}$. This bounds is sharp. Moreover we discuss also the case when $G$ is not connected providing also a sharp lower bound which is linear in both parameters